Hi, welcome to moodle!
I agree that the gradebook versions can be confusing. The gradebook plus v2 is an enhancement of the exisiting one that offers the ability to edit grades directly in the gradebook as well as a couple of other things I'm a bit hazy on. The standard gradebook as far as I know does not allow dual grading like you say, but you could certainly install gradebook plus V2 and keep a single offline assignment separate, to use to keep track of reward points. You would assign them by looking across at the gradebook columns for quiz scores and then altering the totals in the reward points column to refect their efforts.
The gradebook is being overhauled right now and will have a far more flexible and powerful interface in version 1.9 (due in June)
HTH
Matt
Gradebook
Gradebook Confusion
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